add 2nd tab warning

Signed-off-by: szaimen <szaimen@e.mail.de>

php/public/second-tab-warning.js

@@ -0,0 +1,12 @@
+const channel = new BroadcastChannel('tab')
+
+channel.postMessage('second-tab')
+// note that listener is added after posting the message
+
+channel.addEventListener('message', (msg) => {
+    if (msg.data === 'second-tab') {
+        // message received from 2nd tab
+        document.getElementById('overlay').classList.add('loading')
+        alert('Cannot open multiple instances. You can use AIO here by reloading the page.')
+    }
+});